1. Field of the Invention
The present invention relates to a variable power optical system and an imaging device used for a video camera, an electronic still camera and so forth. More particularly, the present invention relates to a variable power optical system and an imaging device equipped with the variable power optical system which is capable of being preferably used as a surveillance camera and also capable of being used in both the visible region and the near infrared region.
2. Description of Related Art
It is general to use surveillance cameras when empty facilities are operated. In many cases, these surveillance cameras take an image by visible rays in the daytime and by near infrared rays at night. Therefore, it is required that these surveillance cameras are capable of being used in both the visible region and the near infrared region. For the above reasons, concerning the lens system, it is necessary that the chromatic aberration is excellently corrected in a region from the visible region to the near infrared region. Concerning the optical system applied to the surveillance cameras, it is necessary to provide a variable power optical system. Further, the optical system is required to be compact and the optical performance is required to be high.
Concerning the variable power optical system, the size of which is reduced, capable of being used in a region from the visible region to the near infrared region, on which a surveillance camera can be mounted, systems disclosed in JP-A-2005-134887 and JP-A-2006-91643 are known. Each optical system described in JP-A-2005-134887 and JP-A-2006-91643 includes: a first lens group having a negative refractive power; a stop; and a second lens group having a positive refractive power, which are arranged in order from the object side. The first lens group includes two meniscus lenses having a negative refractive power and a cemented lens in which a biconcave lens and a positive lens are cemented to each other, which are arranged in order from the object side, and the second lens group includes two single positive lenses which are arranged from the most object side.
In this connection, recently, there is a rising demand for an image of high quality even in the use of surveillance cameras. Especially, a demand for a variable power optical system is increasing which is applicable to cameras having imaging elements, the number of pixels of which is not less than one million, and capable of being used in a region from the visible region to the near infrared region. However, in order to meet the above demands by the conventional optical system, it is necessary to reduce an aperture ratio. In this case, it is difficult to use the optical system under an imaging condition of the use of surveillance cameras in which illuminance is low. It is necessary for the surveillance camera to be capable of specifying an object even in an environment of low illuminance. Therefore, the lens system of the surveillance camera is required to be a fast optical system, the aperture ratio of which is high.